Lens-based high-resolution x-ray microscopy largely resulted from research work at synchrotron radiation facilities in Germany and United States starting in the 1980's. While projection-type x-ray imaging systems with up to micrometer resolution have been widely used since the discovery of x-ray radiation, ones using x-ray lens with sub-100 nanometer (nm) resolution began to enter the market only this century. These high-resolution microscopes are configured similarly to visible-light microscopes with an optical train typically including an x-ray source, condenser, objective lens, and detector.
Because x rays do not refract significantly in most materials, nearly all such x-ray microscopes use diffractive objective lenses, called Fresnel zone plates. As illustrated in FIG. 1, they are essentially circular diffraction gratings, with the grating spacing decreasing with increasing distance from the center in order to increase the diffraction angle and thus produce the focusing effect. By year 2009, x-ray microscopes using synchrotron x-ray sources have achieved 30 nm resolution and commercial systems using laboratory x-ray sources have achieved 50 nm resolution.
Compared with the widely used visible light and electron microscopy techniques, x-ray microscopy combines properties that make it favorable for a large number of applications: (1) high energy x rays have very large penetration length to image internal structures of a thick samples without deprocessing; (2) the absorption and fluorescence emission depends strongly on the elemental composition of the sample, allowing high-sensitivity material analysis; and (3) x-ray imaging causes little structural damage to integrated circuit samples without a charging effect.
The key component of an x-ray microscope is the zone plate lens that focuses the x-rays and magnifies the x-ray images. From FIG. 2, the diffraction-limited resolution of the zone plate lens z is δ=1.22 Δrn, the focal length is f=2rn/(λΔrn), and the numerical aperture is NA=λ/(2Δrn). Zone plates with zones intended primarily to block x-ray radiation are called amplitude zone plate. They can provide up to 9% efficiency. Zone plate with zones intended to produce an ideally π phase shift are called phase zone plates. They can provide up to 40% efficiency. In practice, a zone plate will both absorb and phase shift the x-ray beam impinging on it, and will behave as a combination of amplitude and phase zone plates. Even higher theoretical efficiency can be achieved when the zones approximate the profile of a Fresnel lens. This type of “blazed” zone plates can achieve nearly 100% theoretical efficiency.
The efficiency depends primarily on the wavelength and the thickness of the zones. An amplitude zone plate reaches its maximum efficiency when each zone completely absorbs the x-ray beam; and a phase zone plate reaches its maximum efficiency when each zone shifts the phase of x-ray beam by π, with no absorption. For example, with higher x-ray energy, the zone thickness must be increased to maintain absorption or phase shift.
Thinner zone plates are generally acceptable when using soft x-ray energies within the range of 200-500 eV. However, in order to image inorganic materials such as that used in material science research or semiconductors industries, the x-ray energy must be increased to multi-keV range in order to penetrate samples without excessive deprocessing.
FIG. 3 is a plot of efficiency as a function of x-ray energy for different gold zone plate thicknesses. With higher energy x-ray radiation, thicker zone plates are required to achieve its optimal efficiency. For example, a zone plate having a thickness of 1650 nanometers (nm) reaches a maximum efficiency at just below 10 keV. At this same energy, a 350 nm thick zone plate has an efficiency below 5%. Therefore, the challenge of making high resolution and high efficiency zone plate lenses becomes the challenge of making structures with high thickness-to-width aspect ratio, especially with increasing x-ray energy. For zone plates with 50 nm outer zone width, this would require an aspect ratio of 33. Such a high aspect ratio often poses significant difficulty for fabricating a single optic element.
The criticality in fabricating thicker zone plates comes in the fabrication and the mechanical stabilization of the outer zones. It is here that the aspect ratios become extreme since the outer zones are the narrowest zones, yet have to be the same height as the other, inner, wider zones. Fabricating these zones challenges existing fabrication processes such as plating technology due to the narrowness of the zones. And then, once fabricated, those high aspect ratio zones can be easily toppled by mechanical stress or other stresses due to charging effects.
Some have proposed to fabricate effectively thick zone plates by aligning and stacking separate zone plates to create a compound optic. One specific example relies on the formation of a zone plate doublet by fabricating two zone plates on either side of a common substrate. This approach is problematic, however, because it necessitates thin substrates and front side and backside alignment and fabrication. Moreover, the first fabricated zone plate must survive the fabrication process for the second zone plate. Another approach relies on the fabrication of a series of zone plates successively, one on top of the other. In such approach, however, tolerances stacked up. It further requires effective planarization prior to forming the next zone plate along with techniques for stabilizing the zones sufficiently to survive multiple planarization processes.
Nevertheless, compound x-ray optical elements have been developed. U.S. Pat. No. 6,917,472 B1 describes an Achromatic Fresnel Optic (AFO). This is typically a two element compound optic that is comprised of a diffractive Fresnel zone plate and a one or more refractive Fresnel lenses. Generally, AFO's have been proposed for imaging short wavelength radiation including extreme ultraviolet (EUV) and x-ray radiation. The diffractive element is the primary focusing element, and the refractive element typically provides no or very little net focusing effect. It serves to correct the chromatic aberration of the zone plate.